1. Field of the Invention
The present invention relates to hydraulic valves, and more particularly to devices that regulate pressure at the workports of such valves.
2. Description of the Related Art
Many types of machines have moveable members which are operated by an hydraulic actuator, such as a cylinder and piston arrangement or a hydraulic motor, that is controlled by a valve. The valve has one or two workports connected to the hydraulic actuator and selectively couples each workport to either an outlet of a pump or a fluid reservoir. For example, a three-position, four-way valve has two workports, one connected to the head chamber of a cylinder and piston arrangement and the other to the rod chamber. This valve is moved into different positions to apply pressurized fluid from the pump to first workport and drain fluid from the other workport into the reservoir. Which workport receives pressurized fluid and which workport is coupled to the reservoir determines the direction that the hydraulic actuator moves. Varying the amount that the valve opens controls the rate of fluid flow to and from the workport, thereby proportionally controlling the speed of the hydraulic actuator. The valve has a closed center position at which fluid is neither applied to or drained from the hydraulic actuator.
Damage to the machine and objects nearby can occur if the pressure at a workport exceeds a given level. Individual workport pressures can be limited in a post compensated, closed center valve by using a workport relief valve assembly. A common workport relief valve assembly maintains a spring biased, check valve element against a valve seat on the workport. When the workport pressure becomes greater than the pressure that the spring holding the check valve element closed can support, the workport relief valve opens thereby venting fluid to the reservoir. That venting reduces the pressure at the workport to the desired level.
Although this type of workport relief valve achieves the desired effect, it has an undesirable side affect. Even though the pressure is reduced to the desired level, a large portion of the pump output flow often is consumed by the workport relief valve venting it to the reservoir, which wastes available fluid flow and energy. Any other hydraulic functions being operated on the machine only can receive whatever amount of the pump output flow remains.